This invention relates to hydraulic steering systems having a steering valve that closes the inlet port from a pressure source in the neutral position of its associated valve actuator and is related to the invention disclosed in copending application Ser. Nos. 065,839 and 074,105, filed Aug. 13, 1979 and Sept. 10, 1979, respectively, owned in common by the same assignee.
Steering valves of the foregoing type as compared to those that are open in the neutral position, do not require a continuous flow of the pressure medium so that there are no flow circulation losses while the power steering system is inoperative and less power is thereby utilized. Such steering valves are primarily used for hydro-steering systems having a mechanical connection between the hand steering wheel and a servomotor connected to the vehicle steering linkages and hydraulic assist is required only during vehicle travel along curved paths, but not during straight ahead travel so that a considerable amount of power may be conserved.
In hydrostatic steering systems, a mechanical connection between the hand wheel operating means and the servomotor does not exist. Vehicles so equipped cannot be safely steered at high speeds. However, in such steering systems, loss of power is avoided by circulation of the pressure medium whenever a steering turn remains constant while the vehicle is either at a standstill or in motion.
Steering valve assemblies that block fluid flow from the pressure source in the neutral position of the valve actuator are already known as disclosed, for example, in U.S. Pat. No. 3,855,904 to Jablonsky.
In such prior steering valve assemblies, fluid pressure balanced inlet valves are seated in the neutral position and prevent fluid circulation losses. Whenever the valve-operating hand steering wheel is displaced beyond the limited steering play amount, one of the inlet valves is opened and operating pressure within one of the operating chambers of the servomotor builds up. The inlet valves are so arranged that a feedback force proportional to the servomotor operating pressure may be felt by the vehicle driver on the hand steering wheel to provide the required steering sense. When the steering motion imparted by the driver to the hand steering wheel is terminated, one of the inlet valves is opened rather abruptly through a rigid mechanical connection. As a result, pressurized fluid enters an operating chamber of the servomotor at an excessively high rate to produce flutter in the hydraulic power coupling process. Such flutter could not be heretofore eliminated except by use of a pressure reducer principle involving a rather complex and expensive construction as disclosed, for example in German Patent Document No. OS 12 14 554 to BOSCH.
It is therefore an important object of the present invention to provide a steering valve mechanism having seated inlet valves that block the flow of fluid to the servomotor in the neutral position of the valve actuator and yet prevent development of any flutter. A further object is to provide a steering valve mechanism of the foregoing type through which a feedback reaction from the pressure chamber of the servomotor to the hand wheel operating device is preserved.